This invention relates to adaptation circuit for operating a high-pressure discharge lamp provided with a first and a second input terminal intended for connection to a supply source and with a first and a second output terminal intended for connection of a high-pressure discharge vessel of the high-pressure discharge lamp, each input terminal being connected to the respective output terminal, while the connection between the first input terminal and the first output terminal includes a first controlled semiconductor switch, of which a control electrode is connected to a junction between a first and a second branch of a voltage division circuit, which at least in the case of a connected lamp is arranged parallel to the first semiconductor switch. The invention further relates to a lamp provided with the adaptation circuit.
An adaptation circuit of the kind mentioned in the opening paragraph is known from U.S. Pat. No. 3,925,705. The known circuit permits the operation of a high-pressure discharge lamp in an apparatus provided with a stabilization ballast that is not adapted to the relevant lamp. Thus, besides a continuous improvement of the luminous efficacy of high-pressure discharge lamps, saving of energy can be attained on an existing apparatus while maintaining a desired illumination intensity.
Variations in the voltage of the supply source will lead, when using the known circuit, to variations in the control of the semiconductor switch and accordingly to variations in the lamp current and the lamp power. Variations in a voltage or a current are to be understood herein to mean variations in the value of the root of the time averaged square of the value of the relevant voltage or current, the so-called RMS value. In the case of power, variations are considered with respect to the value averaged in time.
High-pressure discharge lamps in many cases exhibit during their lifetime a variation in lamp voltage, lamp current and lamp power due to ageing processes in the lamp. Variations in lamp properties due to voltage source variations and due to lamp ageing may be disadvantageous on the one hand to the lamp because of increased lamp voltage and on the other hand for the adaptation circuit due to increased lamp current. An increased lamp voltage may lead to the lamp being extinguished because a higher reignition voltage is required at the increased lamp voltage, which may rise above the available supply source voltage. An increased lamp current will result in a larger current flowing through the semiconductor switch and thus leads to a higher dissipation in the semiconductor switch. More particularly, in the case of incorporation of the circuit, for example, in a lamp cap, this may give rise to problems.